Aneuploidy is relatively common in mammalian preimplantation embryos, due to chromosome segregation errors in mitosis. A new paper in Development investigates whether the mitotic timer mechanism, a checkpoint that detects prolonged M phase, which may indicate an unhealthy cell division, is active in preimplantation mouse embryos. We caught up with first author Adélaïde Allais and corresponding author Greg FitzHarris (Professor at the Université de Montréal) to find out more about their research.

Greg FitzHarris (L) and Adélaïde Allais (R)

Greg, can you give us your scientific biography and the questions your lab is trying to answer?

GF: Hi! I was a medical student in the UK, but did a PhD as part of my training. I fell in love with research and so did a postdoc instead of finishing medicine (I also concluded that I'd be a pretty awful medical doctor). I did both my PhD and postdoc on mouse eggs and embryos, and have been working with them ever since. I set up my own lab at University College London in the UK in 2007, then moved the lab to the CRCHUM research centre at the Université de Montréal in 2014. I like to think of our lab as having one foot in cell and developmental biology, and one foot in reproductive medicine. Mouse eggs and embryos are a wonderful model system for addressing questions about cell division that are hard to get at in ‘traditional systems’. But at the same time, most of the projects in the lab are pretty clearly framed around some aspect of what goes on in the clinical IVF environment. We also have some small but very important projects using human oocytes to verify how translatable some of our key findings are to the reality in the clinic.

Mouse eggs and embryos are a wonderful model system for addressing questions about cell division that are hard to get at in ‘traditional systems’

Adélaïde, how did you come to work with Greg and what drives your research today?

AA: We know that infertility affects lot of couples. Since I started my studies, I have always been interested in what can affect female reproduction and how to potentially improve fertility. Greg's laboratory is interested in factors that contribute to embryo aneuploidy, which reduces embryo health and could contribute to pregnancy loss. I joined Greg's lab to try to improve knowledge on how aneuploidy arises at the egg and embryo level, which could help us to develop better methods for selecting the best embryo to transfer back into the patients in the fertility clinic. This could ultimately increase the chance of a successful pregnancy.

Can you give us the key results of the paper in a paragraph?

GF & AA: Cells in general try to avoid aneuploidy, and many mechanisms and checkpoints exist to ensure that aneuploidy is not common. In previous work from the lab, we have begun to show that these checkpoints (the spindle assembly checkpoint and tetraploidy checkpoints, for example) are weak in embryos, and that this might be part of the reason for high levels of embryo aneuploidy. The goal of Adélaïde's study was to specifically interrogate another such mechanism, called the ‘mitotic timer mechanism’. This is a mechanism, identified in somatic cells, that detects cells that take unduly long to divide – which probably indicates a problem – and so stops them from dividing further. In a nutshell, Adélaïde's experiments show that this mechanism doesn't work in embryos and, as a result, embryos are prone to a type of chromosome error called ‘cohesion fatigue’, and that this can probably lead to aneuploidy.

Why do you think M-phase checkpoints are so much weaker in the early embryo compared with somatic cells?

GF & AA: It seems that the mammalian embryo has ‘chosen’ to tolerate a certain amount of aneuploidy, and recent studies in mouse and some clinical data suggest that, in some cases, a mosaic embryo can go on and become a live birth. It is as if the embryo has made a decision to survive rather than focus on quality control. How this all works is not really known. And more broadly, whether there is a physiological benefit to the reproduction of the species of setting the system up like this is something we talk about in the lab a lot, but we are still without a totally compelling answer!

Do you think that maintaining the lagging chromosomes as micronuclei in the embryo, rather than fusing back with the nuclei, as observed in somatic cells, helps with the propagation of aneuploid cells – any idea of the mechanism behind this?

GF & AA: Yes. Previous work from our laboratory showed that micronuclei in mouse embryos are unilaterally inherited at the next cell division, and this unavoidably must promote aneuploidy. It appears this is because the DNA is so heavily damaged in the micronucleus that the chromosomes can no longer form a kinetochore, and so is not recognised and is moved by the spindle in the next cell division. There may be a benefit to this, however, since any chromosome damage that happens within micronucleus is apparently never re-introduced into the genome – unlike that which occurs in chromothripsis in somatic cells. We imagine that these cells are ultimately lost from the embryo, but that's yet to be shown properly.

Impact of prolonged mitosis on the mitotic spindle. Embryos are treated with APCin for 2, 6 and 12 h, respectively.

Impact of prolonged mitosis on the mitotic spindle. Embryos are treated with APCin for 2, 6 and 12 h, respectively.

What impact do you hope will your work have on embryo selection in IVF clinics?

GF & AA: A major innovation in human IVF clinics is that many centres now make ‘timelapse’ movies of developing embryos in the hope of having more information to help select the best embryo for transfer to the patient. This approach is extremely promising, but remains in its infancy and is far from perfected. One possibility is that incorporating information on the duration of M phase (as opposed to just the time between cell divisions) might help identify better embryos, but this is yet to be seen.

When doing the research, did you have any particular result or eureka moment that has stuck with you?

AA: I think every scientist has a eureka moment, we just need to remember it! For me, I think that good results are more exciting when you don't expect them or when you have to wait for those results for a long time. My first breakthrough in this study came at the beginning, when I needed to find a really good way to prolong mitosis reversibly. For months I tested different approaches until I found a great way of doing it without any negative effect on embryo health. At this point, I realised that my project would be possible, and I would be able to answer my question. The second eureka moment was my first live imaging experiment, where I saw those beautiful cells dividing with my own eyes. One of the first things that one of my colleagues told me at the beginning of my PhD was, ‘Working with mouse embryos is challenging, they are fragile and sensitive, but they are amazing cells’. This turned out to be so true.

And what about the flipside: any moments of frustration or despair?

AA: Part of the deal of doing research in science is accepting there will be up and down moments. I think the most frustrating moment is when you have good promising preliminary data that give you interesting results and then on the day of the big experiment you have some technical issues that you had not predicted. And in some cases, you first need to understand what the problem is, so you start the troubleshooting and questioning everything in the laboratory (equipment, yourself…). The good thing about this frustration phase is that you realised how everything can be ‘simple’ when it is working properly!

What next for you after this paper?

AA: We are preparing another paper on a different project, not in embryos but in mouse and human eggs, in collaboration with McGill's fertility clinic. Then, I will prepare to defend my thesis and look for a postdoc position.

Where will this story take your lab next?

GF: We are still quite heavily focussed on why chromosome segregation errors occur in eggs and embryos. However, Adélaïde's paper is one of our first steps toward addressing the flip side of the story – why and how do aneuploid (or otherwise unhealthy) cells choose to continue to divide in the early embryo? This is the sort of question we need to answer in order to help our clinical colleagues in their attempts to improve embryo selection in the clinic.

Finally, let's move outside the lab – what do you like to do in your spare time?

AA: Outside of the lab, I like cooking, hiking, gardening and learning about different cultural traditions.

GF: I play soccer quite a lot still, but it's looking increasingly unlikely I will ever play for England!

A.A. & G.F.: Centre de recherche du Centre Hospitalier de l'Université de Montréal (CRCHUM), H2X 0A9 Montréal, Québec, Canada.

G.F.: Department of OBGYN, and Department of Pathology and Cell Biology, Université de Montréal, H3T 1C5 Montréal, Québec, Canada.


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Absence of a robust mitotic timer mechanism in early preimplantation mouse embryos leads to chromosome instability